In a radio telephone system such as a cellular phone system or a satellite communication system serving a large number of subscribers, or in a hybrid system serving dual mode cellular/satellite portable phones, a calling channel is used to alert a subscriber to an incoming call. The calling channel in a reverse direction is used by the portable phone to reply to a call originated by the cellular network. The reverse calling channel, sometimes known as the random access channel, is also used by portable phones to place calls which they originate. In this case, the forward control channel is used by the network to grant the requested call access to the system by issuing a traffic channel allocation to the portable telephone.
FIG. 1 illustrates 10 cells, C1-C10, of a cellular mobile radio system. For each cell, there is a corresponding base station, B1-B10. The base stations are situated in the center of the cells and have omni-directional antennas. Mobile stations M1-M10 are also shown. The mobile stations may be small lightweight battery powered portable telephones or more bulky vehicle installed telephones which are powered by the vehicles electric power system. The mobile stations may be moved within a cell and from one cell to another. A mobile switching center (MSC) is connected to all of the base stations by cables or any other fixed means, like a radio link. Some of these cables or means are omitted from the figure for simplicity. The MSC is also connected by cables or links to a fixed public telephone network or a similar fixed communication network.
During operation, the mobile station is in contact with the fixed part of the radio system by transmission of radio signals to and reception of radio signals from the different base stations. Telephone calls, data communication links or other communication paths may be set up between a first mobile station and a second mobile station in the system. Telephone calls may also be set up with mobiles in another mobile radio system or with subscribers to the fixed public telephone network. For the purpose of this application, such telephone calls and data communication links are all called connections irrespective of whether they originate in the mobile station or end in a mobile station.
FIG. 1 does not illustrate a complete normal mobile radio communication system but rather only part of a mobile radio communication system. Normally such a mobile radio communication system will comprise more cells and base stations than the 10 cells and base stations illustrated in FIG. 1. In addition, the number of mobile stations will normally be much larger. Some cells of a cellular system may also be served by more than one base station. A complete cellular mobile radio system may also include more MSCs with connected base stations and the mobile stations are usually free to communicate via these additional base stations. In some systems, the base stations are not connected directly to the MSC but rather to a base station controller BSC. In this type of system, a plurality of base stations can be connected to one BSC. A plurality of BSCs are then connected to a mobile switching center.
In FIG. 1, all of the cells have the same hexagonal shape and the same size. Cells therefore, which are not located on the boundary of the radio system, have six adjacent cells, e.g., cell 1 has adjacent cells C2-C7. In practice, cells may have different shapes and are not limited to the hexagonal shape illustrated in FIG. 1. Cells surrounded by other cells may then have more or less than six adjacent cells. In addition, a complete cellular system may also have some larger umbrella cells each covering an area also covered by a group of smaller cells. Base stations located in the vicinity of the cell borders and with sector antennas are also known in the art.
In a system illustrated in FIG. 1, it is desirable that the vast majority of call attempts be successful, for example, 99.9%. Unfortunately, transmission errors can arise that impede a mobile phone or a network from correctly recognizing a call. This can be alleviated by the use of error correction coding in the transmitted signal so that coded calls can be recognized even when transmission errors occur. However, error correction coding expands the general capacity utilized to make each call attempt which in turn reduces the capacity of the calling channel. In addition, no amount of error correction coding per se can overcome the failure of a mobile phone to respond to a call because the mobile phone is temporarily shadowed from the transmitter while passing under a bridge or past a tall building. Therefore, it is necessary to supplement more sophisticated forms of error correction coding with a simple procedure of repeating the call signal after a suitable delay. However, the capacity of the radio system does not normally allow for more than one or two repeats, and timing difficulties in prior art system generally result in the transmission of repeats whether they are needed or not. For example, the U.S. AMPS cellular system transmits five calling message repeats for every call regardless if its answered on the first or second try. Similarly, prior art systems employ error correction coding whether or not the coding is needed for a particular mobile station.